1. Field of the Invention
The invention deals with a capacitive angular displacement transducer for the measurement of a rotor angle. It encompasses a first stator plate having a number of electrodes in the form of conductive circular ring sectors with a sector angle respectively predefined. The transmitting electrodes are electrically isolated from each other and parallel to a second stator comprising a receiving electrode. A rotor is perpendicularly mounted on a shaft and located coaxially, between and parallel to both stator plates. The rotor has at least one rotor blade in the form of a circular ring sector, the central angle of each rotor blade being essentially equal to the sum of the central angles of n neighboring transmitting electrodes. Furthermore, the invention deals with a measurement signal processing device which can be used to determine the angular shaft position by means of the capacitive angular displacement transducer.
2. Description of the Prior Art
Such a capacitive angular displacement transducer and measurement signal processing device results from the applicant""s U.S. Pat. No. 5,598,153, for example. This well known angular sensor features four or eight sector-shaped transmitting electrodes which fully cover a rotational angle of 2xcfx80. The receiving electrode is in the form of a circular ring electrode. The rotor comprises one or two sector-shaped blades, and the shape of each blade has to conform to the rule that the central angle of each rotor blade is equal to the sum of the central angles of two transmitting electrodes of the first stator. The measurement signal processing device of such a capacitive angular transducer includes a generator with four outputs electrically connected to the said four or eight transmitting electrodes of the first stator, as well as an evaluation unit for determining the angular shaft position, connected to the receiving electrode of the second stator. Due to the special geometric properties of the described sensor and to the design of its measurement signal processing unit, a particularly high measurement accuracy is obtained for a measurement range of 360xc2x0, although the design is simple in structure and low in cost. The measurement accuracy is almost unaffected by mechanical tolerances such as rotor offset, tilt and the like.
A tile angle transducer known from DE 44 24 538 A1 is not equipped with a transmitting and a receiving electrode, but with an equally designed stator electrode arrangement, whereby the electrodes of the first stator are electrically connected to the electrodes of the second stator and both stators are electrically conductive. The sensor measures the absolute capacitance value, with the objective to obtain the lowest possible frictional resistance, from a first electrode at the first stator by way of a rotor to a second electrode of said first stator, whereby an identical topology of the second stator with corresponding electrodes is connected in parallel. As the principle of operation is non-radiometric, offset and gain error remain uncompensated as a measuring fault in the measuring chain.
A capacitive linear sensor results from EP 0 538 184 A1, whereby a cursor position is determined relative to e.g. a scale equipped with grooves. Thereby, different electrode arrangements at the cursor are disclosed, particularly an arrangement with side by side transmitting, shielding and receiving electrodes.
One task of this invention lies in the further development of the sensor disclosed in U.S. Pat. No. 5,598,153 and its related measurement signal processing device in order to reduce the amount of space consumed by sensor outlines and to achieve easy and simple mounting of all sensor components. Additionally, easy exchange of specific sensor components should be possible, whereby the simple sensor assembly, the low manufacturing cost and the high measurement accuracy should be maintained.
The task outlined above is accomplished by a sensor of the aforementioned type when at least one stator with an aperture alongside the outer edge is used and the sum of the central angles of all transmitting electrodes is less than 2xcfx80, preferably in the range of xcfx80/12 to 5xcfx80/6, whereby said aperture is chosen to allow during assembly a radial insertion of the shaft along with the rotor to its correct shaft position. The first stator must have a central angle of less than 360xc2x0 and the central angle of the electrode of the second stator has to be essentially equal to the sum of the central angles of all electrodes of the first stator and the electrodes of the first stator include two shielding electrodes. These inventive measures make it possible to manufacture a sensor which is less space consuming. Above all, the aperture enables the mounting and dismounting of both stators independently of the rotor and of each other. As the electrode area is reduced at the stator in question, it is no longer necessary to construct it as a circular ring disk. Although less space is required and the production is simple, it is possible to measure an angular range of 2xcfx80 with high accuracy using a sensor according to the invention, especially as the shielding electrodes largely prevent field deviations caused by auxiliary influences.
If transmitting segments are excited in pairs, it may happen that neighboring electrode pairs do not directly neighbor but lie apart due to their location at the radial edges of the transmitting segments arrangement facing free space. Consequently, the field distribution will be different in the two cases in question. In response to this, an application variant has been developed where the stator has 2n+1 (nxe2x89xa74) transmitting electrodes. Thus, at any time, n predetermined transmitting electrodes can be electrically coupled to n neighboring transmitting electrodes, so that at any time one single transmitting electrode is available without an electrical link to a neighboring transmitting electrode. This arrangement allows paired electrodes to be directly neighboring at all times, so that whichever pairs of transmitting segment couples are excited, field distribution of the same kind for all electrode pairs results.
Furthermore, it was found to be advantageous if the receiving electrode is in the form of a circular ring sector with a central angle essentially equal to the sum of the central angles of all transmitting electrodes. If necessary, the receiving electrode can be completely surrounded by a shielding electrode.
To achieve a measurement range of 2xcfx80 (360xc2x0) for the sensor, one advantageous application uses a rotor with at least two blades in the form of circular ring sectors with an equal central angle. In order to obtain a simple but unequivocal relation between the rotor position and the angular shaft position it may be advantageous to use at least two rotor blades with different radial dimensions, whereby the first stator carries two more electrodes in addition to the transmitting electrodes. While the two electrodes have a distance from the shaft which is greater than the radial dimension of a shorter rotor blade, they are located within the area of the radial dimension of a longer rotor blade. Due to the different dimensions of the rotor blades and the corresponding stator electrodes, additional information is obtained in a simple way which provides a reliable absolute angular rotor position.
It is advantageous for practical use to use identical central angles for all transmitting segments, except for the two boundary electrodes located at both radial edges of the stator plate. Corresponding to the expected field distribution, the central angle of the boundary electrodes can be tuned in such a way that at the receiving electrode, on an average, equally sized induced charges (originating from each single transmitting segment) are obtained. Using the expected field distribution as a guideline for sizing he transmitting electrodes proves to be an effective way to achieve a consistent measurement accuracy over the entire measurement range of the sensor.
The sensor""s measurement accuracy can be improved further if shielding electrodes shaped as circular ring sectors are arranged alongside the boundaries which point to a radial direction of the first stator, and if the central angle of the receiving electrode of the second stator is essentially equal t the sum of the central angles of all transmitting electrodes plus both shielding electrodes. This leads to slightly more space consumption, but results in an optimal shielding of the sensor along its boundary. Consequently, outside effects leading to possible field deviation inside the sensor can largely be avoided. These measures affect neither the simple sensor assembly nor the exchangeability of sensor components.
Two beneficial but nonlimiting applications arise within the context of this invention, for practical purposes. Either the first stator comprises eight or nine transmitting electrodes with a central angle of approximately xcfx80/8 (22.5xc2x0) each and the rotor comprises at least one, preferably two, diametrically opposed rotor blades with a central angle of approximately xcfx80/2 (90xc2x0), or the first status comprises eight or nine transmitting electrodes with a central angle of approximately xcfx80/12 (15xc2x0) each and the rotor comprises at least one, preferably three, equidistantly spaced rotor blades with a central angle of approximately xcfx80/3 (60xc2x0).
In the context of this invention, it should be pointed out that the angular displacement transducer can not only be used to determine both the absolute or relative angular position, but also to determine the angular rate.
We start with an angular displacement transducer of the type described, including a stator with 2n+1 (nxe2x89xa74) transmitting electrodes, from which at any time n predetermined transmitting electrodes can be electrically coupled to n neighboring transmitting electrodes, so that at any time one single transmitting electrode with no electrical link to a neighboring transmitting electrode is available, and including a particularly effective measurement signal processing device which is marked by a combination of the following characteristics: The receiving electrode of the second stator is connected to an evaluation unit capable of processing a single input signal from the receiving electrode, whereby the evaluation unit includes a separation unit to separate the receiving signal with respect to the transmitting signals. A signal processing unit delivers the angular position or the angular rate, and a generator is connected to each single transmitting electrode with one output each. For a predetermined first interval the generator is capable of delivering two identically predetermined first transmitting signals to at least two neighboring transmitting electrodes, and two identically predetermined second transmitting signals to at least two neighboring transmitting electrodes for a predetermined second interval. The transmitting electrode couples belonging to the first transmitting signal are shifted from those transmitting segment couples belonging to the second transmitting signal by at least one electrode. During each of the two intervals at least one of the said transmitting electrodes located at the edge has to be connected to ground.
An additional improvement of the generator and the evaluation unit can be obtained if the generator contains a unit for modulating the transmitting signals with an electrical high frequency signal, and if the evaluation unit contains a carrier frequency amplifier and a demodulator for this high frequency signal. Due to the high frequency modulator (narrow-band system), the sensor is insensitive to disturbing foreign fields and to leakage currents resulting from conductive coatings at the second stator, the receiving disc.
A remarkable improvement of the signal-to-noise ratio can be achieved if a resonant circuit tuned to the carrier frequency is introduced at the receiving electrode of the angular displacement transducer, whereby the capacitor of the resonant circuit includes the capacitance of the receiving electrode.
As a further development, a generator which is able to produce excitation signals between the first interval and the second interval, and after the second interval during a neutral interval can be used in the measurement signal processing device. Such excitation signals should be selected in such a way that an average displacement current is induced at the receiving electrode, largely independent of the angular rotor position. Consequently, a charge amplifier or a carrier frequency amplifier included in the evaluation unit is retracted during intervals during which no measurement activity occurs, to an approximate mid-position, independent of the angular rotor position.
Additional benefits and features of the present invention are noted in the following description of nonrestricting applications of the invention. The description refers to the attached Figures.